VII.] PRESERVATIVES IN DUNG-MAKING 201 



fix the ammonia, and the true antiseptics which will 

 check the production of either ammonia or free nitro- 

 gen gas. Of the first class of substances, the oldest 

 proposal was to use gypsum, which would react with 

 the ammonium carbonate and form the non-volatile 

 ammonium sulphate. 



(NH4)2C03 + CaS04, 2H2O - (NHj2S04-f-CaC03+ 2H2O. 



The drawback to the use of gypsum lies in the large 

 quantities that are required ; the reaction represented 

 by the above equation is really a reversible one, so 

 that only part of the ammonium carbonate is trans- 

 formed into sulphate, the amount being proportional 

 to the excess of gypsum present. As also the gypsum 

 is an insoluble salt, far more than the calculated quan- 

 tity will be required for an efficient fixation of the 

 ammonia. Again, the urine contains nearly all its 

 potash in the form of potassium carbonate, and this 

 also will react with gypsum, increasing the quantity 

 that must be used before the ammonia is fixed. 



From the above equation, about 11 lb. of gypsum 

 would be required for each ton of dung, but at least 

 ten times as much as this would be necessary in practice, 

 or a hundredweight of gypsum, costing 2s. for 

 each ton of farmj'ard manure made. Besides the 

 question of cost, another great drawback to the use 

 of gypsum lies in the fact that the calcium sulphate 

 is itself liable to bacterial change ; during the storage 

 of the dung it is reduced by anaerobic bacteria to the 

 state of calcium sulphide, which afterwards acts injuri- 

 ously on plant life when the farmyard manure is 

 applied to the soil. 



Another suggestion has been to use kainit, because 

 it is composed of salts of magnesium and potassium 

 which will to a certain extent be transformed into 



